On Board Diagnostics

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On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since the introduction in the early 1980's of on-board vehicle computers, which made OBD possible. Early instances of OBD would simply illuminate a malfunction indicator light, or MIL, if a problem were detected—but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized fast digital communications port to provide myriad realtime data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.

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OBD-II

OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signalling protocols available, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. Finally, the OBD-II standard provides an extensible list of DTCs. As a result of this standardization, a single device can query the on-board computer(s) in any vehicle. This simplification of reporting diagnostic data led the feasibility of the comprehensive emissions testing program envisioned by the CARB.

Hyundai Error Codes

The OBD-II Diagnostic Connector

The OBD-II specification provides for a standarized hardware interface—the female 16-pin (2x8) J1962 connector. Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, the OBD-II connector is always located on the driver's side of the passenger compartment near the center console. SAE J1962 defines the pinout of the connector as:

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Bus positive Line of SAE-J1850

-

Chassis ground

Signal ground

CAN high (ISO 15765-4 and SAE-J2234)

K line of ISO 9141-2 and ISO 14230-4

-

-

Bus negative Line of SAE-J1850

-

-

-

CAN low (ISO 15765-4 and SAE-J2234)

L line of ISO 9141-2 and ISO 14230-4

Battery voltage

The assignment of unspecified pins is left to the vehicle manufacturer's discretion.

OBD-II Signal Protocols

There are five signalling protocols currently in use with the OBD-II interface. Any given vehicle will likely only implement one of the protocols. Often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:

ISO 9141-2. This protocol has a data rate of 10.4 kbaud, and is similar to RS-232. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.

pin 7: K-line

pin 15: L-line (optional)

UART signaling (though not RS-232 voltage levels)

K-line idles high

High voltage is Vbatt

Message length is restricted to 11 bytes, including CRC

ISO 14230 KWP2000 (Keyword Protocol 2000)

pin 7: K-line

pin 15: L-line (optional)

Physical layer identical to ISO 9141-2

Data rate 1.2 to 10.4 kbaud

Message may contain up to 255 bytes in the data field

ISO 15765 CAN (250kbit/sec or 500kbit/sec). The CAN protocol is a popular standard outside of the automotive industry and is making significant in-roads into the OBD-II market share. By 2008, all vehicles sold in the US will be required to implement the CAN bus, thus eliminating the ambiguity of the existing five signalling protocols.

pin 6: CAN High

pin 14: CAN Low

Note that pins 4 (battery ground) and 16 (battery positive) are present in all configurations. Also, ISO 9141 and ISO 14230 use the same pinout, thus you cannot distinguish between the two simply by examining the connector.

Diagnostic data available via OBD-II

OBD-II provides access to numerous data from the ECU and offers a valuable source of information when troubleshooting problems inside a vehicle. The SAE J1979 standard defines a method for requesting various diagnostic data and a list of standard parameters that might be available from the ECU. The various parameters that are available are addressed by "parameter identification numbers" or PIDs which are defined in J1979. For a list of basic PIDs, their definitions, and the formulae to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs. Manufacturers are not required to implement all PIDs listed in J1979 and they are allowed to include proprietary PIDs that are not listed. The PID request and data retrieval system gives access to real time performance data as well as flagged DTCs. For a list of generic OBD-II DTCs suggested by the SAE, see Table of OBD-II Codes. Individual manufactures often enhance the OBD-II code set with additional proprietary DTCs.

OBDII Scan Tools

OBDII scan tools can be categorized in two ways, based on whether they require a computer to operate (stand-alone vs PC-based), and the intended market (professional or hobby/consumer use).

PC-Based Scan Tools

The advantages of PC-based scan tools are:

Low cost (compared to stand-alone scan tools with similar functionality)

Virtually unlimited storage capacity for data logging and other functions

The Future of On-Board Diagnostics

An OBD-III specification is in the regulatory development phase. Information on the content of this specification is limited. Some have speculated that OBD-III will include the capability for a vehicle to report emissions violations automatically using some sort of radio transmitter.